This invention relates to biological filter structures.
Volatile organic compounds (VOCs) of various kinds are widely used in many industrial processes. Typical VOCs include acetone, benzene, toluene, xylene and methyl ethyl ketone (MEK). Disposal of VOCs or substances containing VOCs represents a significant problem for many industries.
A known technique for disposal and/or destruction of VOCs from certain sources is to allow any volatilizable compounds present in the materials to evaporate at ambient temperature into an effectively unconfined space. Since the VOCs evaporate at a fairly slow rate, and the space into which the VOCs evaporate is effectively unconfined, the concentration of VOCs in the atmosphere in the vicinity of the residue material does not normally become bothersome, but this practice nevertheless contributes to atmospheric pollution.
U.S. Pat. No. 5,518,920 discloses apparatus for capturing air that contains VOCs in the vapor phase and for processing the air in order to convert the VOCs to a nontoxic form. In particular, U.S. Pat. No. 5,518,920 discloses a biological filter unit comprising a cabinet containing biologically active filter material. The filter material includes plant compost containing microorganisms that have an affinity for VOCs and consume VOCs as nutrients. The cabinet has an inlet opening at the bottom for supply of feed air and an exhaust opening at the top. Feed air containing VOCs is supplied to the inlet opening and flows upwards through the filter. The microorganisms capture and consume the VOCs, and accordingly, under appropriate operating conditions, the concentration of VOCs in the exhaust air is substantially less than the concentration of VOCs in the feed air.
In order to provide the filter unit with sufficient resilience to withstand heavy loadings of VOCs, it is necessary that the filter unit should contain a large population of microorganisms, which in turn requires a large quantity of compost. In the filter unit disclosed in U.S. Pat. No. 5,518,920, the compost is supported in generally horizontal beds. Cost considerations dictate that the number of beds be kept to a minimum, and consequently in order to provide the required quantity of compost, the beds are fairly thick. Specifically, the beds may have a depth of up to about twelve inches.
It is believed that a major part of the biological activity of a biofilter takes place at the surface of the mass of filter material, where the biomatrix is exposed to the incoming pollutant-laden air. Accordingly, it is desirable to maximize the ratio of the surface area of the filter bed to the volume of compost material. A possible disadvantage of the biological filter unit disclosed in U.S. Pat. No. 5,518,920 is that the surface area of the compost beds relative to the volume of the beds is rather small. A further possible disadvantage is that a thick bed of biofilter material results in a rather large back pressure, requiring use of a large amount of energy to maintain the flow of air through the filter bed.
In accordance with a first aspect of the invention there is provided an article of manufacture comprising a hollow cartridge for holding compost in a biofilter, the cartridge having two ends spaced apart along an axis of the cartridge, the cartridge being substantially a regular polygon in cross section perpendicular to the axis of the cartridge and having a side wall at each side of the polygonal cross section, the side walls being formed with openings that are small enough to substantially retain the compost in the cartridge yet large enough to allow filaments of bacteria present in the compost to project through the openings, the polygonal cross-sectional form of the cartridge being such that the cartridge can be placed side-by-side with a plurality of other substantially identical cartridges in an array such that each side wall of the cartridge is in parallel confronting relationship with a side wall of one of the other cartridges, and a first side wall of the cartridge being provided with a projecting baffle structure which holds the first side wall away from a confronting side wall of an adjacent cartridge in such an array, the baffle structure defining a serpentine path for flow of air between said first side wall of the cartridge and the confronting side wall of the adjacent cartridge.
In accordance with a second aspect of the invention there is provided an article of manufacture comprising a hollow cartridge for holding compost in a biofilter, the cartridge having two ends spaced apart along an axis of the cartridge, the cartridge being substantially rectangular in cross section perpendicular to said axis and having four side walls, the side walls being formed with openings that are small enough to substantially retain the compost in the cartridge yet large enough to allow filaments of bacteria present in the compost to project through the openings, and a first side wall of the cartridge being generally flat and a second side wall being provided with a projecting baffle structure which defines a serpentine path over said second side wall.
In accordance with a third aspect of the invention there is provided an assembly of substantially identical hollow cartridges for holding compost in a biofilter, each cartridge having two ends spaced apart along an axis of the cartridge and being substantially a regular polygon in cross section perpendicular to the axis of the cartridge and having a side wall at each side of the polygonal cross section, the side walls being formed with openings that are small enough to substantially retain the compost in the cartridge yet large enough to allow filaments of bacteria present in the compost to project through the openings, the polygonal cross-sectional form of the cartridge being such that the cartridges can be placed side-by-side in an array such that each side wall of a first cartridge is in parallel confronting relationship with a side wall of one of the other cartridges.
In accordance with a fourth aspect of the invention there is provided an assembly of substantially identical cartridges for holding compost in a biofilter, wherein each cartridge has two ends spaced apart along an axis of the cartridge and is substantially uniform in cross section perpendicular to the axis of the cartridge, has an endless sidewall formed with openings that are small enough to substantially retain the compost in the cartridge yet large enough to allow filaments of bacteria present in the compost to project through the openings, and is of cross-sectional form such that the cartridges can be placed side-by-side in an array with passages between each cartridge and adjacent cartridges.